With more optical communication networks having a higher speed and a higher capacity, increasing demands have been placed on optical signal processing devices for performing the processing of Wavelength Division Multiplexing (WDM) optical signals. For example, a multiplexed optical signal at a node is not subjected to an optical to electrical conversion but the optical signal is directly subjected to a path switching. Thus, transparent optical signal processing devices have been promoted.
On the other hand, from the viewpoint of the downsizing and integration of an optical signal processing device, Planar Lightwave Circuits (PLC) have been researched and developed. The PLC is structured so that a silicon substrate for example has thereon a core made of silica glass to integrate various functions in one PLC chip, thus realizing an optical functional device having a small loss and high reliability. Furthermore, a complex optical signal processing devices appeared that is a combination of a plurality of PLC chips and other optical functional devices.
For example, Patent Publication 1 discloses an optical signal processing device that is a combination of a Planar Lightwave Circuit (PLC) including an Arrayed Waveguide Grating (AWG) for example and a spatial light modulator such as a liquid crystal device. More particularly, a wavelength blocker consisting of a PLC and a collimating lens symmetrically provided to sandwich a liquid crystal device at the center, a wavelength equalizer and a dispersion compensator for example have been examined. In these optical signal processing devices, a plurality of optical signals having different wavelengths are independently subjected to an optical signal processing for each wavelength.
FIG. 6 is a schematic diagram illustrating one example of an optical signal processing device. In this optical signal processing device, an optical signal is inputted and outputted via a spectroscopic element 51. The spectroscopic element 51 demultiplexes a WDM optical signal by outputting the signal at an outgoing angle θ depending on the wavelength thereof. The demultiplexed optical signal is outputted to a focusing lens 52. The optical signal focused by the focusing lens 52 is focused, in accordance with the outgoing angle θ, at each focusing point at a predetermined position of the signal processing device 53 having a function of intensity modulation or phase modulation or beam steering. Specifically, it is noted that the optical signal is focused at a different position of the signal processing device depending on the wavelength of the input optical signal. The signal processing device 53 is a liquid crystal device consisting of a plurality of element devices (pixels) for example. By the control of the transmittance of each element device for example, an optical signal of each wavelength is subjected to intensity modulation for example, thereby realizing a predetermined optical signal processing function. The optical signal subjected to the optical signal processing is reflected by a mirror 54 to thereby invert the traveling direction. Then, the optical signal passes through the focusing lens 52 and is multiplex again by the spectroscopic element 51. As is commonly well known, the spectroscopic element 51 also can multiplex an optical signal by changing the traveling direction of the optical signal to a direction opposite to that during demultiplexing. A multiplexed optical signal of each wavelength is outputted again to the outside of the optical signal processing device as output light.
In FIG. 6, the spectroscopic element 51 is schematically shown and may be any spectroscopic element so long as the spectroscopic element 51 can demultiplex and multiplex an optical signal depending on the wavelength of the optical signal. For example, the spectroscopic element may be a grating, a prism, or Arrayed Waveguide Grating (AWG) for example. The signal processing device may be the one that can modulate the intensity or phase of an optical signal, that can modulate the intensity and phase, or that can subject the traveling direction of an optical signal to a beam steering. For example, the signal processing device includes a liquid crystal device, a Micro Electro Mechanical Systems (MEMS) mirror, and optical crystal for example. The optical crystal may be any one including electrical optical crystal typical example of which is LiNbO3 so long as the optical crystal can achieve a desired modulation.
The optical signal processing device shown in FIG. 6 has a configuration in which a mirror is used to reciprocate an optical signal so that a single spectroscopic element can demultiplex and multiplex the optical signal. This configuration is generally called a reflection type. A device that performs an optical signal processing such as a wavelength blocking is not limited to this configuration. For example, another configuration is also possible in which, without using the mirror of FIG. 6, signal processing devices are positioned on symmetry planes and an outgoing system consisting of another one lens and a spectroscopic element is provided at a position that is on an extended line of an incident light path axis and that is symmetric to the incoming system with regard to the symmetry plane. This configuration is a configuration where the demultiplexing and multiplexing of an optical signal are performed respectively via independent incoming system outgoing system and is called a transmission type. Furthermore, by changing the direction of the mirror in the device configuration of FIG. 6, another configuration is also possible where an outgoing system that is provided at an arbitrary position and that consists of another one lens and a spectroscopic element is used to multiples an optical signal. For example, another configuration is also possible where the reflecting plane of a mirror is inclined by 45 degrees to the incident light path of an optical signal and a lens provided in a vertical direction to the incident light path and a spectroscopic element are used to configure an outgoing system. Another configuration also can include a plurality of outgoing systems when the signal processing device has a beam steering function.
In FIG. 6, the spectroscopic element 51 and the focusing lens 52 are provided to have a Front Focal Length (FFL) therebetween. The signal processing device 53 and the focusing lens 52 are provided to have a Back Focal Length (BFL) therebetween. The focal point of light focused by the focusing lens 52 must on a surface of the mirror 54 at all wavelengths used. Displacement of the focal point from the mirror surface causes a disadvantage of an increased coupling loss of light. Another disadvantage is that the focused optical signal has an increased beam spot diameter, thus causing a disadvantage of a decreased wavelength resolution.
Furthermore, the signal processing device 53 must include a spatially-periodic structure in order to selectively modulate an optical signal in accordance with each wavelength. For example, when the signal processing device 53 is a liquid crystal device, the liquid crystal device must have an element device that has a structure designed in accordance with the optical characteristic of the spectroscopic element and the focusing lens.
More specifically, it is known that the wavelength dependency of the focusing position on a signal processing device follows a value obtained by multiplying an angular dispersion value of a spectroscopic element with a focal length of a focusing lens. The wavelength dependency of the focusing position is also called a linear dispersion value of a spectroscopic optical system. The linear dispersion value of an optical system determined by a spectroscopic element and a focusing lens must sufficiently match the linear dispersion value used for the design of the signal processing device. Any displacement between these linear dispersion values causes an unmatching between the position of the focusing point of an actual optical signal and the positions of the individual element devices of the signal processing device (e.g., pixels of a liquid crystal shutter device), thus failing to perform a desired processing.
Patent Publication 1: Japanese Laid-Open Publication No. 2002-250828 (page 16, page 19, FIG. 20, FIG. 27, FIG. 29D for example) Patent Publication 2: Japanese Laid-Open Publication No. 2001-255424 Non-Patent Publication 1: H. Takenouchi, T. Ishii, T. Goh, “8 THz bandwidth dispersion-slope compensator module for multiband 40 Gbit/s WDM transmission system using an AWG and spatial phase filter”, Electronics Letters, Vol. 37, No. 12, pp. 777-778, 2001 Non-Patent Publication 2: K. Li et al., “Coherent micromirror arrays,”, Optics Letters, Vol. 27, No. 5, pp. 366-368, 2002